The in vitro metabolism of phencyclidine by liver preparations of the rat and rabbit are being studied to determine the pathways that lead from administered drug to eliminated product. The intermediate metabolites could be responsible for some of the pharmacological actions of the original substance and/or retained by tissues to cause toxicological effects. In the study an equimolar mixture of 2H5-phencyclidine and 2Ho-phencyclidine was incubated with liver microsome preparation, then the mixture extracted with an organic solvent, and extract examined by GC/MS. Metabolites resulting from phencyclidine were identified by M and M plus 5 doublets in their mass spectra. Using this technique, metabolites in which the piperidine ring, cyclohexyl ring and both alicyclic rings are hydroxylated were identified. Using standards available from NIDA, a quantitative assay was developed for the three metabolites and phencyclidine itself. The results indicated that phencyclidine is extensively metabolized and that the metabolite assayed accounted for less than 50% of the total material present. Quantitative differences were between the two species and a new metabolite was characterized but not identified.